I have tried installing Cyanogenmod 10 on my Samsung Rugby Smart AT&T, but now I want to revert back to my old factory ROM. But how?
If you don't have a Nandroid backup, you'll have to find a stock ROM for your device. Your device is not that well-known, the only resource I could find is on XDA-Developers. There you can find information on how to use ODIN to flash a stock ROM (post #5).
In the future, creating a Nandroid backup is always recommended before flashing a ROM, just in case something happens or you want to go back.